1. Introduction
This invention relates to removal of contaminants from organic solutions. In a preferred embodiment, this invention relates to removal of anions such as halide ions from photoresist compositions.
2. Description of the Prior Art
Photoresists are light-sensitive compositions used for the formation of images in the manufacture of electronic devices. Photoresist coating compositions typically comprise a light-sensitive component and a polymer binder dissolved in a solvent. Typical photoresist compositions are disclosed in U.S. Pat. Nos. 5,178,986; 5,212,046; 5,216,111; and 5,238,776, each incorporated herein by reference for disclosure of photoresist compositions, processing, and use.
In the manufacture of high resolution integrated circuits, it is known that many processing liquids come into contact with a bare wafer or a resist coated surface. These include the resist itself and treatment chemicals such as organic liquids and aqueous solutions which contain acids, bases, oxidants, and other proprietary ingredients. At least 15 to 50 liquids of various compositions are used to clean wafers, prime surfaces, deposit resists or other polymers, develop, rinse, etch, and strip the resist. It is known that these solutions are a source of contaminants that can interfere with the performance of the integrated circuit. Thus, the reduction or removal of insoluble and soluble contaminants from processing fluids used for the production of integrated circuits before or during use is basic insurance for prevention of damage to the integrated circuit.
Photoresist liquids are known to contain particulate and ionic contaminants. For example, it is known that solid gels or insolubles form in photoresists due to dark reactions. In addition, soluble impurities such as moisture, silicone oils, plasticizers, and metal ions may be present from the manufacture of the resist components and from the packaging containers or dispensing tanks. Trapped bubbles from point-of-use filtration or the shaking of a resist bottle prior to dispensing can lead to defects in resist coatings. In Class 100 clean rooms, airborne particulate counts amount to 3 particles per liter of density of 2. By comparison, liquids contain about 100,000 particles per liter. A particle count of 100,000 per liter seems high, but if translated into a solid of 0.6.mu. in size (entity of 2), this is equivalent to 10 parts per million (ppm). A level of 10 ppm amounts to the deposition of 10 mg per liter. Since liquids are heavily contaminated compared to clean room air, effective contaminant removal is essential to the manufacture of such devices.
Ultrafiltration of resist liquids has progressed and manufacturers of resist now supply resist materials filtered through 0.04 .mu.M diameter absolute filters. Other methods for removal of particulates such as gels include ultracentrifugation, electrostatic treatment of the resist, and depth filtration. These methods are useful for the removal of particulates but are not effective in removing dissolved contaminants such as organic impurities and ionic species. These contaminants can be as damaging to an integrated circuit as particulate contamination.
Dissolved contaminants in resists such as metal ions, organic contaminants and halide ions require more sophisticated detection and removal methods than the methods used to remove particulates. One such method is disclosed in International Publication No. WO 93/12152 which is directed to removing metal ions such as sodium and iron from novolak resins during manufacture. The process comprises cation exchange treatment whereby a cation exchange resin is first washed with a mineral acid solution to reduce the level of total sodium and iron ions within the exchange resin to preferably less than 100 ppb, passing a formaldehyde reactant through the so treated cation exchange resin to decrease the sodium and iron ion content to less than 40 ppb, passing a phenolic compound through the cation exchange resin to decrease its sodium and iron ion content to less than 30 ppb, and then condensing the so treated phenolic compound with formaldehyde in the presence of an acid catalyst to form the resin. This method of removal is cumbersome, does not remove contaminants from other photoresist components and does not remove the ionic species from the photoresist that enters between the time of manufacture of the phenolic and the use of the photoresist.
A method for removal of organic contaminants together with chelated multivalent metal ions is disclosed in copending U.S. patent application Ser. No. (Attorney Docket No. 42602), filed Oct. 20, 1993, assigned to the same assignee as the subject application and incorporated herein by reference. In accordance with the procedures of said copending application, multivalent metal ions are first chelated with a chelating agent for the metal ions, preferably a chelating agent that is an organic impurity in the photoresist formulation such as an azo or diazo dye. The solution of the photoresist containing the chelated, multivalent metal ion is then contacted with a pure activated carbon which selectively removes organic contaminants from the solution of the photoresist including the organic contaminants chelated with the multivalent metal ions.
A method for removal of ionic metals from a photoresist is disclosed in published Japanese patent application No. 1228560 published Sep. 12, 1989, incorporated herein by reference. In accordance with the procedures of this patent, a photosensitive resin is passed through a mixed bed of a cation exchange resin and an anion exchange resin. Though the process of the reference does remove the contaminates as described therein, it has been found that the acid proton or the cation exchange resin may attack acid labile components of a photoresist, such as solvents and resins conventionally used in photoresist compositions with the formation of undesired by-products. It has also been found that the strong base of the anion exchange resin may attack base labile materials such as o-quinonediazide photoactive components of a photoresist with the formation of undesired by-products. Consequently, though the process of the reference removes ionic metals, the procedure introduces other undesirable by-products into the photoresist formulation.
In copending U.S. patent application Ser. No. 08/128,994, filed Sep. 30, 1993, assigned to the same assignee as the subject application and incorporated herein by reference, a process is disclosed for removing metallic ions from organic solutions using modified cation exchange resins. In accordance with the process of the invention disclosed therein, the cation exchange resin is modified by replacement of the acid protons on the cation exchange groups with essentially neutral groups such as ammonium or amine groups. Thereafter, an organic solution containing acid labile components may be treated with the modified cation exchange resin to remove metal ions without the formation of undesired by-products caused by attack of acid protons on acid labile groups.